The invention relates to thermoplastic multi-layer barrier packaging films and bags or pouches made therefrom. In particular, this invention relates to films and bags having heat shrink and cold seal properties, that give good seal strength and wherein the outer layer comprises nylon directly adhered to a barrier layer comprising a polyvinylidene chloride copolymer.
Shrinkable and heat sealable thermoplastic barrier films are being used in the packaging of non-food products as well as the packaging of food products such as meat, cheese, poultry and the like.
A laminate film known from U.S. Pat. No. 3,741,253 comprises a core layer of a vinylidene chloride copolymer (i.e. saran, a well known barrier material) between a layer of a cross-linked ethylene-vinyl acetate copolymer (EVA) used as an inner sealant layer and an outer layer of ethylene-vinyl acetate copolymer which is not cross-linked.
As disclosed in U.S. Pat. No. 4,064,296 the core layer may also be a hydrolized ethylene-vinyl acetate copolymer (EVOH). EVOH is also known as vinyl alcohol polymer and is commercially available from Kuraray Company of Japan as EVAL. It has oxygen barrier properties similar to those of saran, i.e. copolymers of polyvinylidene chloride (PVDC), and offers the advantage that it may be irradiated without discoloration.
Many attempts have been made with barrier films to combine good shrink properties with satisfactory strength, toughness, and sealability while simplifying processing and saving processing costs by eliminating adhesive interlamination between a pair of adjacent layers. Nylon is known for its toughness. When one of the layers of a barrier film is an EVOH barrier material and another is nylon, it is already known from U.S. Pat. No. 4,355,721 (1982) (assignee: American Can) that EVOH directly adheres well to nylon. This patent relates to a heat sealed package for commercial sterilization and unrefrigerated storage of food products wherein the multilayer polymeric material of the package has an interior oxygen barrier layer consisting essentially of a vinyl alcohol polymer which is essentially free of acetal groups. Furthermore, that nylon directly adhered to EVOH may be employed in a heat shrinkable (oriented film) is also known from U.S. Pat. No. 4,501,798 (1985) (assignee: American Can). But there is still room for improvement in having a pair of adjacent layers in direct contact with each other when one layer comprises nylon (also herein referred to as polyamide, sometimes abbreviated as PA) and the other barrier layer comprises a polyvinylidene chloride copolymer.
As is discussed in copending U.S. Ser. No. 911,936, commonly assigned to W. R. Grace and Co., filed Sept. 26, 1986, the disclosure of which is incorporated herein by reference, more recently low density linear polyethylenes have become commercially available and have also been proposed for packaging applications. Actually the so called linear low density polyethylenes are copolymers of ethylene and varying amounts of higher alpha-olefins with e.g. 5 to 10 carbon atoms per molecule (U.S. Pat. No. 4,076,698) or 3 to 8 carbon atoms per molecule (published European patent application No. 120503). Depending on their density these materials are referred to as linear low density polyethylene (LLDPE) or very low density polyethylene (VLDPE), the separation line being at a density of about 0.910 g/cm.sup.3. Some properties of VLDPE have been described in Plastics Technology, September 1984, page 113 and October 1984, page 13 as well as in a company brochure published in February 1984 by DSM in the Netherlands and entitled "Stamilex PE". Their properties are said to be a unique combination between those of standard polyethylene and polyolefinic rubbers. Their sealability and their compatibility with other polymers has been mentioned.
The gist of U.S. Ser. No. 911,936 is to provide a material combining improved shrinkability, increased maximum shrink temperature, good cold seal strength and sealability through contamination. This is achieved by a thermoplastic multi-layer barrier packaging film having an inner heat sealing layer comprising a copolymer of ethylene and x to y% by weight, based on the copolymer, of an alpha-olefin with 4 to 8 carbon atoms, whereby x=0.5n.sub.C +8 and y=1.25 n.sub.C +15 with n.sub.C being the number of carbon atoms in the alpha-olefin, said copolymer having a density of less than 0.920 g/cc. Preferably the inner heat sealing layer comprises a copolymer of ethylene and 12 to 20% by weight, based on the copolymer, of an alpha-olefin with 4 to 8 carbon atoms. Although in terms of domination, the scope of the claimed invention in U.S. Ser. No. 911,936 would not exclude a PVDC layer directly adhered to a nylon layer, the laboratory Examples of U.S. Ser. No. 911,936 wherein the outside layer comprises polyamide (PA) (reproduced below as a Comparative Example) always show the PA adhered to the adjacent oxygen barrier PVDC layer with an adhesive, namely acid modified EVA. Therefore, what is positively taught is use of an adhesive. It has been surprisingly found by the present inventor that the nylon "outside" layer embodiment of the films according to U.S. Ser. No. 911,936, which films have the improved properties mentioned above because of the "inner" heat sealing layer being a linear copolymer of ethylene and alpha-olefin having a density less than about 0.920 g/cm.sup.3, can be made without the need for the acid-modified EVA adhesive to provide inter layer adherence between the PA layer and the PVDC barrier layer. Moreover, other barrier films wherein the "inner" sealing layer is other than that according to U.S. Ser. No. 911,936, (for instance, sealing layers of other materials such as an inner layer of EVA, an inner layer of LLDPE having a density greater than 0.920, etc.) can be made in accordance with the present invention having an outer layer comprising nylon wherein the nylon is directly adhered to the adjacent PVDC barrier layer without the need for any adhesive therebetween.